Many digital systems require a backup power supply for instances where main power becomes unavailable. Typically this has been done using batteries, but with the development of very high value capacitors (supercapacitors) it is quite often preferable to replace a battery with a capacitor. This is done mainly for service reasons: supercapacitors can endure more charge/discharge cycles than rechargeable batteries, and have a longer useable life than batteries leading to reduced service needs for a given product requiring a backup mechanism.
Known backup power mechanisms using supercapacitors for energy storage comprise two separate circuits: a circuit to charge the supercapacitor when a main power supply is available, and a switching power supply running off the supercapacitor when the main power supply is unavailable.
A simple example of a backup power mechanism with separate charge and discharge circuits is presented in FIG. 1. When the main power supply (not shown) is available, Vcc is generated by this power supply. During this time, a switch 102 is closed allowing a supercapacitor 104 to charge via a current source 103. The current source 103 may include a resistor, active current source, switching supply or other mechanism. A switch 106 is open during charging. The switch 102 is modulated to maintain a fixed (maximum) voltage on the supercapacitor 104. This will generally be performed by a control mechanism (not shown).
When the main power source is lost, the switch 102 is opened and the switch 106 is modulated to transfer energy from the supercapacitor 104 to Vcc via an inductor 108 and a diode 110. Output filtering is performed by output capacitors of the main power supply (not shown). Thus there are separate charge and discharge circuits. This use of separate circuits for charge and discharge requires additional part count thereby adding cost, Printed Circuit Board (PCB) layout area and weight.
A higher efficiency can be achieved when the diode 110 has a switch across it to form a synchronous rectifier. A circuit having this additional component is shown in FIG. 2. A switch 202 is connected in parallel with the diode 110. However, the circuit of FIG. 2 has a separate charge and discharge circuit.
There are supercapacitor charging schemes of the art that only provide for simple charging mechanisms where the supercapacitor is placed directly across the voltage allowing a very large current at the start of charging.
There is therefore a need to provide a supercapacitor based backup power system that minimizes part count, provides efficient output voltage generation and provides controlled (the instantaneous current requirements of the voltage source are limited) and power-efficient charging of the supercapacitor.